The present invention relates to a vertical round container for storing liquids, the container having a floating cover that extends over and above the surface of the liquid with the cover sealed in relation to the container wall by means of flexible sealing elements. The floating cover has at least one compartment on its lower side which is open towards the liquid and in which gaseous media/vapors are entrapped. Sensing devices are provided for determining the immersion depth of the cover, and supply and discharge lines are provided for varying the quantity of the gaseous medium in the compartments.
A floating cover of this type is known from the published European Patent No. EP 0 059 298 A1.
Floating covers on liquid surfaces function to suppress the exchange between the two fluid media, the liquid and gas, in storage containers.
An important application of such floating covers is presented by large-volume storage containers for highly volatile mineral oil products. To solve the main problems—to prevent fire and explosion and to avoid emissions—the floating cover was developed approximately 80 years ago.
Following this development two main systems were pursued; namely, a tank or container with a floating cover without a fixed roof (also called an “external floating roof tank”) and a tank with a fixed roof and an internally located floating cover (also called an “internal floating roof tank”).
Since the problems created by residual emissions and precipitations in a container without fixed roof have not been solved in a satisfactory way, preference has primarily been given to containers with fixed roofs. The possibility has here been seen to dissolve emissions almost completely with the help of vapor balancing with corresponding containers and by vapor recovery and to dispense with the floating cover in the end.
The resulting technique, however, has decisive drawbacks.
All of the completely closed containers containing hydrocarbon vapors present a high risk of explosion and fire and are sensitive to vacuum and overpressure. The daily tank breathing by expanding and contracting gases in the container headspaces requires additional gas containers and the treatment of large gas volumes.
Vapor recovery is expensive and energy-consuming, and the total emission balance becomes negative as compared to containers with a floating cover.
It has therefore been evident that the most expedient methods for minimizing facility risks, total emissions and financial expenditure in storing highly volatile and combustible liquids, make use of the floating cover; consequently, this technique has not been abandoned. Ideally, the exchange balance between liquid and vapor must be established underneath the floating cover in closed chambers, so that evaporation comes to a standstill. However, this ideal case cannot be maintained permanently due to the necessary operating processes in the container and the presently used constructional solutions.
In practice, welded constructions of steel are exclusively used for external floating covers for refinery products. There are various other constructions for internal floating covers in the form of aluminum member structures with vapor chambers between membrane and liquid level and plastic membranes with direct contact with the liquid.
The above-mentioned EP 0 059298 A1 describes a floating cover for liquid containers which comprises a steel membrane slightly inclined towards the center, a wall on the peripheral side that extends from the plane of the membrane both upwards and downwards, with vapor cushions formed between the membrane and the liquid level underneath the membrane through a plurality of vertical partitions, these vapor cushions carrying the floating roof. The immersion depth of the floating roof is measured, and the floating position of the roof is trimmed by adding compressed air into individual buoyancy compartments. Excessive vapor or gas quantities underneath the roof are discharged into the atmosphere. In this floating roof, which has the form of a pantile construction with vapor cushions and which has a trim device for maintaining the horizontal float position, there is no pronounced water collection zone. This means that the water can settle on the roof to a minor extent, requiring frequent trimming with a supply of air underneath the membrane and later vapor discharge directly to the atmosphere. The entry of air or vapors with the filling of the container also requires the subsequent discharge of vapors into the atmosphere.
All of these trimming operations cause major emissions. In addition, there are the unsolved emission problems during filling and emptying of the container. Consequently, a floating cover of this type has not found practical application in the industry.